Active substrates for color displays

ABSTRACT

An improved electronic display that includes components selected to enhance display performance. The improved display includes an active substrate that has a plurality of thin film transistors and a plurality of thermally transferred color filters that include a colorant in a crosslinked binder. The active substrate can also include a black matrix. Other components in the improved display such as a liquid crystal material, spacers, and bottom polarizer, can be selected to enhance display performance characteristics such as brightness, power consumption, response time, weight, and thickness. The invention also provides a method of forming a color filter substrate for displays including the steps of thermally mass transferring a plurality of color filters and crosslinking the plurality of color filters after transfer. Before the crosslinking step, the plurality of color filters can be inspected and removed for reworking of the substrate.

[0001] The present invention pertains to improved color electronicdisplays, to active substrates for use in electronic displays, and tomethods of making active elements that have color filters for use inelectronic displays.

BACKGROUND

[0002] Difficulties can arise in the fabrication of full color activematrix liquid crystal displays (LCD), especially as demand increases fordisplays to become larger, brighter, thinner, and lighter, with higherresolution and faster switching times, all at a lower cost.

[0003] Active matrix LCDs commonly include a liquid crystal materialdisposed between an active substrate and a non-active (passive)substrate. The active substrate generally has a regular array, ormatrix, of thin film transistors (TFT) arranged in pixels. To add color,color filters can be provided for each of the pixels in an LCD.Currently, color filters in active matrix displays are typically formedonto the flat, non-active display substrates, using eitherphotolithography or direct printing techniques.

SUMMARY

[0004] Increasing the size and/or the resolution of electronic displaysalso increases the number of pixels to control in the display. With morepixels to control, issues regarding electronic switching speed, liquidcrystal response times, and color filter alignment can become moreimportant. For example, when assembling color LCDs, improperly alignedor otherwise defective color filters can necessitate discarding thecolor filter substrate during processing. The associated cost of waste,as well as the probability for error, tends to increase as the number ofpixels in the display increases. In addition, there is an ever-growingdesire for these displays to have thinner and lighter weightconstructions, to use less power while providing enhanced brightness,and to have improved reliability and durability.

[0005] The present invention provides improved liquid crystal displayconstructions that have components selected to enhance overall displayperformance. Components can be selected to construct improved displaysaccording to the present invention that have, for example, enhancedbrightness, lower power consumption, and/or faster switching rates. Thepresent invention also includes improved displays that can be madethinner and lighter while maintaining reliability and durability.

[0006] According to the present invention, the following components canbe selected to achieve enhanced display performance. Color filters canbe provided on the active substrate of an active matrix display, forexample, to increase the aperture ratio of the display, resulting inmore light through the display. Liquid crystal materials and alignmentlayers can be selected to increase response times, reduce powerconsumption, and increase contrast. Spacers can be provided that allowfor thinner constructions that are durable. Reflective polarizers can beused that increase lighting efficiency by increasing brightness for thesame lighting conditions. These and other components can be selectedindividually or in combination, for example to synergistically enhanceone or more display performance properties.

[0007] In accordance with an aspect of the present invention there isprovided an improved color active-matrix liquid crystal display. Oneembodiment provides an electronic display that includes (i) a lightsource; (ii) a polarizer disposed to transmit and substantially polarizelight from the light source, and (iii) a liquid crystal display paneldisposed to utilize light transmitted by the polarizer. The displaypanel includes a bottom substrate, a top substrate spaced a distanceapart from the bottom substrate, a liquid crystal layer disposed betweenthe substrates, and a multi-color active layer disposed between thebottom substrate and the liquid crystal layer. The multi-color activelayer includes a plurality of independently addressable active elementselectrically connected to transparent conductive sub-pixel elements anda plurality of color filters, each aligned with one or more of thetransparent conductive sub-pixel elements. The color filters comprise acolorant in a crosslinked composition and are derived from a thermallytransferred material. The liquid crystal layer can comprise afluorinated chiral ferroelectric liquid crystal material.

[0008] In another embodiment, the present invention provides a coloractive-matrix liquid crystal display that has a first transparentsubstrate having a plurality of independently addressable thin filmtransistors disposed thereon, each transistor electrically connected toan associated transparent conductive sub-pixel element; a secondtransparent substrate spaced a distance apart from the first transparentsubstrate; a liquid crystal layer disposed between the first substrateand the second substrate; and a plurality of color filters disposedbetween the first substrate and the liquid crystal layer, the colorfilters comprising a thermally transferred material having a colorantdisposed therein, each color filter aligned with one or more of thetransparent conductive elements associated with the transistors.

[0009] One component that can be selected to improve performance indisplays of the present invention is color filters. Accordingly, thepresent invention provides a new method of making color filters. In oneaspect, multiple color filters can be transferred prior to crosslinkingto allow inspection and removal (if necessary) of the color filters forreworking of the display substrate.

[0010] In one embodiment, the present invention provides a new method ofmaking color filters for liquid crystal display substrates, includingthe steps of providing a display substrate, thermally mass transferringa plurality of color filters to selected portions of the substrate, eachcolor filter comprising a colorant in a crosslinkable composition, andcrosslinking the color filters after the transferring step. Aftertransferring the plurality of color filters to the display substrate andprior to crosslinking, the color filters can optionally be inspected,for example, to determine if defects are present and if alignment isproper. If the filters do not meet inspection criteria, they can beremoved by a washing step. After washing, the display substrate can bereworked by transferring another plurality of color filters.

[0011] The method of the present invention is particularly suited forthermally transferring a plurality of color filters to an activesubstrate for color active matrix displays. In this case, an activesubstrate that has a plurality of independently addressable activedevices can be provided as the display substrate for thermallytransferring color filters according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic cross-section of a liquid crystal display200.

[0013]FIG. 2(a) is a schematic plan view of an active substrate element300 for electronic displays.

[0014]FIG. 2(b) is a schematic cross-sectional view of a liquid crystaldisplay panel 310 that includes an embodiment of active substrateelement 300 viewed along line 2 b from FIG. 2(a).

[0015]FIG. 3 is a schematic flow chart of steps in a method of thepresent invention.

DETAILED DESCRIPTION

[0016] The present invention pertains to improved active matrix colordisplays and to a method of making color display substrates. Theimproved displays include components selected to enhance the performanceof active matrix displays. The improved displays can also include colorfilter substrates made according to a method of the present invention.The method includes thermally transferring multiple color filters to adisplay substrate and crosslinking the color filters after transfer.

[0017] Liquid crystal light modulators are commonly used in electronicdisplays where pixilated information is to be visually displayed to auser or an observer. Other uses for liquid crystal light modulatorsinclude data transmission devices where information is transmitted aslight which can be controlled by a liquid crystal light modulator. Inthe case of data transmission, light can be “displayed” from one deviceto another device, and is not necessarily displayed directly to a useror observer. As used herein, therefore, the terms “liquid crystaldisplay”, “electronic display”, and “liquid crystal light modulator” aremeant to encompass electronic devices that modulate light transmissionfor display, whether that display is to a passive observer, an activeuser, an active or passive device, photographic film, a projectionscreen, and the like.

[0018]FIG. 1 schematically shows components that can be included in animproved display 200 according to the present invention. In variousembodiments, these and other components (as described in more detailbelow) can be selected (individually or in combination) to improvedisplay performance characteristics, including enhanced brightness,lower power consumption, faster switching times, and thinner overallconstruction. In brief, selectable components of display 200 can includebottom polarizer 210, bottom substrate 212, active layer 214, colorfilter layer 216, bottom alignment layer 218, liquid crystal layer 220,top alignment layer 222, sustain electrode layer 224, top substrate 226,and top polarizer 228. As used throughout this document, the term“active layer” refers to a layer in an electronic display constructionthat includes one or more active devices, and “active substrate” refersto a substrate that has an active layer. Examples of active devicesinclude transistors, counters, gates, and other such devices, whereasexamples of passive devices include capacitors, resistors, and othersuch devices. These and other layers and components includeable in adisplay according to the present invention are described in more detailin discussions that follow.

[0019]FIG. 1 also shows internal light source 202 that can be providedto illuminate display 200. While display 200 is shown to be atransmissive display, it is recognized that the various aspects of thepresent invention also apply to transflective and reflective displaysthat may be illuminated from the front using ambient light or aninternal light source and front light guide.

[0020] Display 200 can optionally include various optical componentsand/or light management films positioned between light source 202 andpolarizer 210, as indicated by region 240 in FIG. 1. The components canbe provided alone or in combination to improve the overall performanceof the display. Examples of suitable components include: a sheet ofBRIGHTNESS ENHANCEMENT FILM (BEF) (commercially available from MinnesotaMining and Manufacturing Company, St. Paul, Minn.) used in a prisms upconfiguration, a prisms down configuration, and/or in combination withanother sheet of BEF (e.g., two sheets oriented in a crossed prismsconfiguration); a sheet of a light management film sold under the tradedesignation DBEF (commercially available from Minnesota Mining andManufacturing Company); a turning film or other light direction changingfilm; a cholesteric reflective polarizer (such as those commerciallyavailable from Nitto Denko under the trade designation NIPPOX); diffusereflective polarizing films such as those described in U.S. Pat. No.5,783,120 and International Publication WO 97/32223; retarder films;lenses and lens arrays; and other such films and optical components.

[0021] One embodiment of an improved display of the present invention isa color filter active substrate element 300 as shown in FIG. 2(a) thatcan be used, for example, in an active matrix LCD construction 310 asshown in FIG. 2(b). The particular embodiments are meant to beillustrative and not to limit the scope of the invention. The variouscomponents shown in FIGS. 2(a) and (b), and the selection of suchcomponents for improved overall display performance, are described inmore detail below after a brief identification of various componentsshown.

[0022]FIG. 2(a) shows a plan view of a portion of an active substrateelement 300 for an electronic display panel. Substrate element 300 canbe integrated, using proper components, into transmissive,transflective, or reflective electronic displays that can range in sizefrom microdisplays (on the order of 1 cm or less), to small displays(about 1 to several cm), to large displays (tens or several tens of cm),and to very large displays (on the order of 1 m or more). Substrateelement 300 has a plurality of independently addressable devices 20,such as thin film transistors (TFTs), arranged in a regular pattern. Thetransistors form an active layer along with address lines and other(optional) active or passive devices that may be provided, includingadditional transistors, capacitors, resistors, organic or inorganiclight emitting devices, and other such devices. The TFTs 20 are commonlyprovided in a rectangular array as shown in FIG. 2(a), although they canbe arranged in any suitable manner. Transparent sub-pixel electrodes 12can be associated with the transistors 20 to allow independentapplication of an electric field in each sub-pixel area to re-orientliquid crystal material in selected sub-pixels of the display. Colorfilters R, G, and B can be provided on the active substrate element 300using the method of the present invention, as described in more detailbelow.

[0023] An individual transistor 20 can be addressed by supplying avoltage to an associated gate electrode 22 from one of a series ofparallel gate driver electrodes 32 so that a signal supplied from one ofa series of parallel signal driver electrodes 34 can pass from thesource 24 of the transistor 20 to the drain 26 of the transistor 20.Drain 26 can be electrically connected to an associated transparentsub-pixel electrode 12. If any layers separate transparent sub-pixelelectrode 12 and drain 26, the electrical connection can optionally bemade via a through hole 28 in the intervening layer or layers. By soaddressing a transistor, an electric field can be created in an areabetween the transistor's associated transparent sub-pixel electrode andan electrode supplied on an opposing substrate. The area associated witheach transparent sub-pixel electrode defines a sub-pixel in the display.To add color, a color filter can be provided for each sub-pixel. In afull color display, a full pixel is made up of three adjacentsub-pixels, each having a different color filter, typically red, blue,and green (although other color combinations can be used, such as cyan,magenta, and yellow).

[0024]FIG. 2(b) shows a portion of a liquid crystal display panel 310that includes an embodiment of the active substrate element 300 shown inFIG. 2(a). Display panel 310 is shown in cross-section taken along line2 b from FIG. 2(a). The electronic display panel 310 includes bottom(active) substrate 10 a, top (non-active) substrate 10 b, and a varietyof components sandwiched between the substrates. Bottom substrate 10 aincludes a series of independently addressable TFT devices 20 that areaddressable via signal driver electrodes 34 electrically connected tosource electrodes 24 and gate driver electrodes (not shown) electricallyconnected to gate electrodes 22. Using high conductivity driverelectrodes with TFTs can allow for faster switching times. Color filtersR, G, and B, can be disposed on the TFTs. To prevent ambient light fromundesirably inducing activation of the TFTs, an optional black matrixmaterial 50 can also be deposited on top of the TFTs, using methodsdescribed in more detail below. Black matrix 50 can also be deposited onthe active substrate to delineate sub-pixels and provide for highercontrast. Planarization layer 16 can optionally be provided to give auniform surface on which to apply transparent sub-pixel electrodes 12.

[0025] Through holes 28 can be provided in the planarization layer 16and color filters R, G, and B, to allow electrical connection betweenTFT drain electrodes 26 and transparent conductive electrodes 12 asshown in FIG. 2(b). Alignment layer 18 a can be provided over thetransparent conductive electrodes to orient the liquid crystal materialused in the display. Top substrate 10 b can also be provided with analignment layer 18 b, which can be similar or dissimilar to alignmentlayer 18 a, depending on the desired liquid crystal alignment. Spacers42 can be provided to maintain a uniform gap between the top and bottomsubstrates, and a liquid crystal material 40 can disposed in the gapbetween the substrates. Depending on how spacers 42 are provided, anamount of residual binder material 44 can be associated with thespacers. As described below, spacers can be selectively deposited toform a black matrix, for example, positioned directly above the TFTs toprotect the transistors from undesired light-induced activation.

[0026] Top substrate 10 b has a transparent electrode 12′, commonlycalled a sustain electrode. Electrode 12′ serves as a counter electrodeto the sub-pixel electrodes 12. For example, sustain electrode 12′ canbe held at a constant potential so that applying an appropriate voltageto a selected sub-pixel electrode or electrodes creates an electricfield in the selected sub-pixel area(s) to orient a liquid crystalmaterial for transmission of light having a desired polarization state.Sustain electrode 12′ can be patterned or provided as a singlecontinuous layer.

[0027] The discussion that follows describes in further detail variouscomponents of improved displays of the present invention as introducedabove and as shown in FIGS. 1, and 2(a) and (b).

[0028] In one embodiment, the performance of a display of the presentinvention can be improved by forming color filters on the activesubstrate of an active matrix display. Referring to FIG. 1, color filterlayer 216 can be provided on the active layer 214 using methods inaccordance with the present invention, as described in detail below.Color filters can be deposited as separate filters for each sub-pixel,or in parallel strips that span multiple sub-pixels, for example, asindicated by color filters R, G, and B in FIGS. 2(a). In either case,the color filters can be arranged so that adjacent filters includedifferent colorants. A typical arrangement has a repeating pattern ofred, green, and blue color filters, or other suitable combination ofcolor filters (e.g., other primary colorants such as magenta, cyan, andyellow, or other color combinations that may or may not provide adisplay that has full color capabilities). Other repeating patterns alsoexist, and the particular pattern used typically depends on thearrangement of active devices and sub-pixel electrodes on the activesubstrate.

[0029] When color filters that contain transparent primary colorants aresuitably arranged, proper activation of adjacent sub-pixels allowsmixing of colors for a full color display. Color mixing can also includethe introduction of gray scale. Gray scale can be introduced by spatialor temporal dithering of the electronic signal controlling the sub-pixelelectrodes (as is well known in the art), or by providing a liquidcrystal material that provides continuous gray scale as a function ofapplied field (e.g. for twisted nematic and super twisted nematic liquidcrystal materials as well as for certain ferroelectric liquid crystalssuch as those discussed in U.S. Pat. No. 5,062,691). The combination ofprimary color mixing and the introduction of gray scale gives theability to achieve a full color electronic display.

[0030] The performance of displays of the present invention can beenhanced by providing color filters on the active layer, for example, toincrease the aperture ratio of the display relative to other displaysthat have color filters disposed on the opposing, passive substrate.Increasing the aperture ratio increases the brightness of the display.When color filters are provided in an active matrix display on thepassive substrate, precise alignment can be difficult to achieve between(1) the pattern of color filters and the black matrix on the passivesubstrate and (2) the pattern of active devices, sub-pixel electrodes,and optional black matrix on the active substrate. To compensate for thedifficulties that arise in precision alignment, the black matrix lineson the passive color filter substrate are commonly widened to ensure acertain amount of overlap between the patterns on each substrate. Thepatterns are overlapped to help prevent cross over of light from onesub-pixel to an adjacent sub-pixel, which can cause discoloration in thedisplay. However, widening of the black matrix also tends to reduce theaperture ratio of the display and, in turn, reduces the amount of lighttransmitted through the display. By way of the present invention, thecolor filter layer can be provided on the active layer rather than onthe passive substrate. This construction can be used to eliminate thecolor filter and the (widened) black matrix from the passive substrate,and thereby increase the aperture ratio and brightness of the display.By providing color filters on the active substrate, the aperture ratioof the display can be increased by 10% to 30% or more relative toconstructions that have color filters and black matrix on the passivesubstrate.

[0031] Color filters can be provided on the active substrate between thesubstrate and the active layer, within the active layer, or between theactive layer and the liquid crystal layer. The color filters can beprovided on active substrates using any suitable method now known orlater developed. Suitable methods can include conventionalphotolithography, thermal head or ink jet printing, colorantsublimation, and selective thermal mass transfer. Preferably, the colorfilters are formed by selective thermal transfer of a colorant. Anexemplary method of selective thermal transfer of a colorant is laserinduced thermal mass transfer of a colorant from a donor sheet to adisplay substrate, as described in detail below.

[0032] Thermal mass transfer occurs, for example, when portions of adonor element transfer layer are thermally transferred to a receptor sothat the transferred portions remain substantially intact, that is thematerial within the transferred portions remains in substantially thesame physical arrangement as prior to transfer. For example, thermalmass transfer includes thermal melt stick type transfers where the heatgenerated in the donor element is sufficient to cause portions of thetransfer layer to detach at the interface between the transfer layer andthe donor element support so that the detached portions of the transferlayer can be transferred as “plugs” onto a receptor. Thermal masstransfer may be a preferred transfer method as compared to other methodsidentified above, for example, due to high placement accuracies (i.e.,when a laser is used to induce the thermal mass transfer), applicabilityto large and small displays as well as to high and low resolutiondisplays, and compatibility with systems that use thermal mass transferto provide other components (e.g., black matrix or spacers, as discussedbelow).

[0033]FIG. 3 indicates steps for transferring multiple color filters todisplay substrates according to the present invention. A first colorfilter material can be selectively transferred to portions of a displaysubstrate by thermal transferring a color filter material to theselected portions of the substrate. A second color filter material cansimilarly be selectively transferred to portions of the substrate. Thisprocedure can optionally be repeated for a third (or more) color filtermaterials.

[0034] After selectively transferring two or more color filtermaterials, the color filters disposed on the active substrate canoptionally be inspected for defects, alignment, and so forth. After anoptional inspection, the color filters can be crosslinked, for example,by radiation curing, thermal curing, or exposure to chemical curatives.Crosslinking hardens the color filter material on the substrate, therebymaking the color filters more chemically, physically, and/or thermallystable, and thus less susceptible to damage that can be caused by laterprocessing or operation. However, crosslinking can also make removal ofthe color filter material more difficult. Therefore, prior tocrosslinking and after an optional inspection, the present inventionincludes an alternative step of removing the color filters prior tocrosslinking for reworking of the active substrate, as indicated in FIG.3. The ability to successfully deposit multiple color filters beforecrosslinking or setting any one color filter provides the advantage thatmultiple color filters can be inspected at the same time, removed at thesame time (if removal is necessary), and crosslinked at the same time.

[0035] Exemplary processes for selective thermal transferring of colorfilter materials include thermal mass transfer, ink jet printing, andcolorant sublimation. Thermal mass transfer techniques include theimagewise transfer of color filter material from a donor sheet byselectively exposing the donor sheet to imaging radiation (e.g., from alaser or a flash lamp), and selective direct heating of portions of thedonor sheet, for example by using a thermal print head or by directresistance heating of a layer contacting or disposed in the donorelement. Colorant sublimation techniques include selective dyesublimation from areas of a donor sheet. Ink jet techniques includeforming a directed flow of a dye or pigment through an aperture ornozzle using heat or otherwise.

[0036] Methods for imagewise thermal mass transfer of color filtermaterials are discussed, for example, in U.S. Pat. Nos. 5,521,035 (Wolk)and 5,725,989 (Chang). Briefly, these processes involve thermallytransferring material from selected areas of a color donor elementsheet. Suitable donor elements are described in detail below. Heat canbe generated using imaging radiation from a laser, flash lamp, or othersuitable source to irradiate selected portions of a light-to-heatconverter (LTHC) material in the donor element. The LTHC can be presentas a separate layer in the donor media and/or included in other layersof the donor media, including in a color filter transfer layer.

[0037] During imaging, the color transfer layer of the donor element canbe brought into intimate contact with a receptor (e.g., an activedisplay substrate that has a plurality of devices, a passive displaysubstrate that has one or more addressable electrodes, or other suitabledisplay substrates). Pressure or vacuum can optionally be used to securethe donor element to the display substrate receptor. The radiationsource can be used to heat the LTHC in an imagewise fashion (e.g.,digitally, analog exposure through a mask, etc.) to cause imagewisetransfer of portions of a color thermal transfer layer from the donor tothe receptor. A variety of light-emitting sources can be utilized.Infrared, visible, and ultraviolet lasers are particularly useful whenusing digital imaging techniques. When analog techniques are used (e.g.,exposure through a mask) high powered light sources (e.g., xenon flashlamps, etc.) are also useful.

[0038] During imaging, it may be desirable to minimize formation ofinterference patterns due to multiple reflections from the imagedmaterial. This can be accomplished by various methods. A common methodis to effectively roughen the surface of the donor element on the scaleof the incident radiation as described in U.S. Pat. No. 5,089,372. Thishas the effect of disrupting the spatial coherence of the incidentradiation, thus minimizing self interference. An alternate method is toemploy an antireflection coating within the donor element. The generaluse of anti-reflection coatings is known, and may consist ofquarter-wave thicknesses of a coating such as magnesium fluoride, asdescribed in U.S. Pat No. 5,171,650.

[0039] Large donor elements can be used, including donor elements thathave length and width dimensions of a meter or more. In operation, alaser can be rastered or otherwise moved across the donor element, thelaser being selectively operated to illuminate portions of the donorelement according to a desired pattern. Alternatively, the laser may bestationary and the donor element and/or receptor substrate moved beneaththe laser.

[0040] In the present invention, it may be desirable and/or convenientto sequentially use two or more different color donor elements to form aplurality of color filters on a display substrate. Alternatively, asingle donor element (e.g., one that has multiple different color filtermaterials patterned to form the color transfer layer) can be used totransfer multiple color filters from the donor element.

[0041] As an alternative to using imaging radiation, a heating element,such as a resistive heating element, may be used to selectively transferportions of the color transfer layer. The donor element can beselectively contacted with a heating element to cause thermal transferof a portion of the transfer layer according to a pattern. In anotherembodiment, the donor element may include a layer that can convert anelectrical current applied to the layer into heat.

[0042] Ink jet printing can also be useful in selective thermal transferof color filters to active display substrates. Methods for ink jetprinting of color filters are described in U.S. Pat. Nos. 5,714,195 and5,716,740. Exemplary color filter materials transferable using ink jetmethods include an ink and a crosslinkable binder. The ink can includeany suitable colorants such as dyes and pigments (as discussed above),and the ink can be liquid or solid. The binder can be any suitablebinder that is ink compatible, examples of which include acrylic resins,epoxy resins, silicone resins, cellulose derivatives such ashydroxypropyl cellulose, hydroxyethyl cellulose, methyl cellulose, andcarboxymethyl cellulose, and modified resins thereof. The color filtermaterial can optionally include additives such as cure initiators,dispersing agents, surfactants, and other suitable additives.

[0043] For embodiments in which color filters are selectively thermallymass transferred from a donor element, suitable donor elements that canbe used typically include a base substrate layer, alight-to-heat-converter (LTHC), a color transfer layer that includes acolorant dispersed in a crosslinkable composition, an optionalinterlayer disposed between the base layer and the color transfer layer,and an optional transfer assist layer.

[0044] The donor substrate can be a polymer film. One suitable type ofpolymer film is a polyester film, for example, polyethyleneterephthalate or polyethylene naphthalate films. However, other filmswith sufficient optical properties (if light is used for heating andtransfer), including high transmission of light at a particularwavelength, as well as sufficient mechanical and thermal stability forthe particular application, can be used. The donor substrate, in atleast some instances, is flat so that uniform coatings can be formed.The donor substrate is also typically selected from materials thatremain stable despite heating of the LTHC during thermal transfer. Thetypical thickness of the donor substrate ranges from 0.025 to 0.15 mm,preferably 0.05 to 0.1 mm, although thicker or thinner donor substratesmay be used.

[0045] Typically, the materials used to form the donor substrate and theLTHC layer are selected to improve adhesion between the LTHC layer andthe donor substrate. An optional priming layer can be used to increaseuniformity during the coating of subsequent layers and also increase theinterlayer bonding strength between the LTHC layer and the donorsubstrate. One example of a suitable substrate with primer layer isavailable from Teijin Ltd. (Product No. HPE100, Osaka, Japan).

[0046] For radiation-induced thermal transfer, a separate LTHC layer istypically incorporated within the donor element to couple the energy oflight radiated from a light-emitting source into the donor element. TheLTHC layer preferably includes a radiation absorber that absorbsincident radiation (e.g., laser light) and converts at least a portionof the incident radiation into heat to enable transfer of the transferlayer from the donor element to the receptor. In some embodiments, thereis no separate LTHC layer and, instead, the radiation absorber isdisposed in another layer of the donor element, such as the donorsubstrate or the transfer layer. In other embodiments, the donor elementincludes an LTHC layer and also includes additional radiationabsorber(s) disposed in one or more of the other layers of the donorelement, such as, for example, the donor substrate or the transferlayer. In yet other embodiments, the donor element does not include anLTHC layer or radiation absorber and the transfer layer is transferredusing a heating element that contacts the donor element.

[0047] Typically, the radiation absorber in the LTHC layer (or otherlayers) absorbs light in the infrared, visible, and/or ultravioletregions of the electromagnetic spectrum and converts the absorbedradiation into heat. The radiation absorber is typically highlyabsorptive of the selected imaging radiation, providing a LTHC layerwith an optical density at the wavelength of the imaging radiation inthe range of about 0.2 to 3, or from about 0.5 to 2.5. (Optical densityis the logarithm of the ratio of a) the intensity of light incident onthe layer and b) the intensity of light transmitted through the layer.)Thicker coatings generally have higher optical densities, but may, atleast in some instances, have less efficient heat transfer. Higheroptical density LTHC layers may provide improved surface topography ofthe transferred layer (i.e., a smoother top surface), but may alsoresult in increased edge roughness of the transferred layer.

[0048] Suitable radiation absorbing materials can include, for example,dyes (e.g., visible dyes, ultraviolet dyes, infrared dyes, fluorescentdyes, and radiation-polarizing dyes), pigments, metals, metal compounds,metal films, and other suitable absorbing materials. Examples ofsuitable radiation absorbers includes carbon black, metal oxides, andmetal sulfides. One example of a suitable LTHC layer can include apigment, such as carbon black, and a binder, such as an organic polymer.The amount of carbon black may range, for example, from 2 to 50 wt. % or5 to 20 wt. %. High carbon black loadings may improve sensitivity andedge roughness of the transferred layer, but may also degrade thesurface topography of the transferred color filter (e.g., due toexcessive heating of the transfer layer during imaging). A suitable LTHClayer formulation is given in Table I. TABLE I LTHC Coating FormulationComponent Wt. % Raven ™ 760 Ultra carbon black pigment 8.0 (availablefrom Columbian Chemicals, Atlanta, GA) Butvar ™ B-98 (polyvinylbutyralresin, 1.4 available from Monsanto, St. Louis, MO) Joncryl ™ 67 (acrylicresin, 4.2 available from S. C. Johnson & Son, Racine, WI) Elvacite ™2669 (acrylic resin, 31.8 available from ICI Acrylics, Wilmington, DE)Disperbyk ™ 161 (dispersing aid, 0.7 available from Byk Chemie,Wallingford, CT) FC-430 ™ (fluorochemical surfactant, 0.03 availablefrom 3M, St. Paul, MN) Ebecryl ™ 629 (epoxy novolac acrylate, 47.6available from UCB Radcure, N. Augusta, SC) Irgacure ™ 369 (photocuringagent, 5.3 available from Ciba Specialty Chemicals, Tarrytown, NY)Irgacure ™ 184 (photocuring agent, 0.8 available from Ciba SpecialtyChemicals, Tarrytown, NY)

[0049] In a particular embodiment, LTHC layers can be used that containconductive materials dissipate charge built up during the process ofcoating layers to form the donor element. For example, LTHC layers canbe used that contain carbon black in a binder that has ionic functionalgroups. LTHC layers so formulated that have conductivities of about 10⁷Ω/square or more may provide adequate anti-static properties. An exampleof such a formulation includes components as shown in Table II. TABLE IILTHC Coating Formulation Parts by Material Supplier Function weightKetjen Black EC600JD Ketjen International Carbon Black 100.0 Disperbyk161 BYK Chemie Dispersant 73.02 UR8300 Toyobo Co. Binder 500.4 EvecrylEB629 UCB Radcure, Inc. Binder 542.0 Irgacure 369 Ciba-Geigy Initiator40.6 Irgacure 184 Ciba-Geigy Initiator 6.0

[0050] Another suitable LTHC layer includes metal or metal/metal oxideformed as a thin film, for example, black aluminum (i.e., a partiallyoxidized aluminum having a black visual appearance). Metallic and metalcompound films may be formed by techniques, such as, for example,sputtering and evaporative deposition. Particulate coatings may beformed using a binder and any suitable dry or wet coating techniques.

[0051] Dyes suitable for use as radiation absorbers in a LTHC layer maybe present in particulate form, dissolved in a binder material, or atleast partially dispersed in a binder material. When dispersedparticulate radiation absorbers are used, the particle size can be, atleast in some instances, about 10 μm or less, and may be about 1 μm orless. Suitable dyes include those dyes that absorb in the IR region ofthe spectrum. Examples of such dyes may be found in Matsuoka, M.,“Infrared Absorbing Materials”, Plenum Press, New York, 1990; Matsuoka,M., Absorption Spectra of Dyes for Diode Lasers, Bunshin Publishing Co.,Tokyo, 1990, U.S. Pat. Nos. 4,722,583; 4,833,124; 4,912,083; 4,942,141;4,948,776; 4,948,778; 4,950,639; 4,940,640; 4,952,552; 5,023,229;5,024,990; 5,156,938; 5,286,604; 5,340,699; 5,351,617; 5,360,694; and5,401,607; European Patent Nos. 321,923 and 568,993; and Beilo, K. A. etal., J. Chem. Soc., Chem. Commun., 1993, 452-454 (1993). IR absorbersmarketed by Glendale Protective Technologies, Inc., Lakeland, Fla.,under the trade designation CYASORB IR-99, IR-126 and IR-165 may also beused. A specific dye may be chosen based on factors such as, solubilityin, and compatibility with, a specific binder and/or coating solvent, aswell as the wavelength range of absorption.

[0052] Pigmentary materials may also be used in the LTHC layer asradiation absorbers. Examples of suitable pigments include carbon blackand graphite, as well as phthalocyanines, nickel dithiolenes, and otherpigments described in U.S. Pat. Nos. 5,166,024 and 5,351,617.Additionally, black azo pigments based on copper or chromium complexesof, for example, pyrazolone yellow, dianisidine red, and nickel azoyellow can be useful. Inorganic pigments can also be used, including,for example, oxides and sulfides of metals such as aluminum, bismuth,tin, indium, zinc, titanium, chromium, molybdenum, tungsten, cobalt,iridium, nickel, palladium, platinum, copper, silver, gold, zirconium,iron, lead, and tellurium. Metal borides, carbides, nitrides,carbonitrides, bronze-structured oxides, and oxides structurally relatedto the bronze family (e.g., WO_(2.9)) may also be used.

[0053] Metal radiation absorbers may be used, either in the form ofparticles, as described for instance in U.S. Pat. No. 4,252,671, or asfilms, as disclosed in U.S. Pat. No. 5,256,506. Suitable metals include,for example, aluminum, bismuth, tin, indium, tellurium and zinc.

[0054] As indicated, a particulate radiation absorber may be disposed ina binder. The weight percent of the radiation absorber in the coating,excluding the solvent in the calculation of weight percent, is generallyfrom 1 wt. % to 30 wt. %, preferably from 3 wt. % to 20 wt. %, and mostpreferably from 5 wt. % to 15 wt. %, depending on the particularradiation absorber(s) and binder(s) used in the LTHC.

[0055] Suitable binders for use in the LTHC layer include film-formingpolymers, such as, for example, phenolic resins (e.g., novolak andresole resins), polyvinyl butyral resins, polyvinyl acetates, polyvinylacetals, polyvinylidene chlorides, polyacrylates, cellulosic ethers andesters, nitrocelluloses, polycarbonates, and acrylic and methacrylicco-polymers. Suitable binders may include monomers, oligomers, orpolymers that have been or can be polymerized or crosslinked. In someembodiments, the binder is primarily formed using a coating ofcrosslinkable monomers and/or oligomers with optional polymer. When apolymer is used in the binder, the binder includes 1 to 50% polymer bynon-volatile weight, preferably, 10 to 45% polymer by non-volatileweight.

[0056] Upon coating on the donor substrate, the monomers, oligomers, andpolymers are crosslinked to form the LTHC. In some instances, ifcrosslinking of the LTHC layer is too low, the LTHC layer may be damagedby the heat and/or permit the transfer of a portion of the LTHC layer tothe receptor with the transfer layer.

[0057] The inclusion of a thermoplastic resin (e.g., polymer) mayimprove, in at least some instances, the performance (e.g., transferproperties and/or coatability) of the LTHC layer. It is thought that athermoplastic resin may improve the adhesion of the LTHC layer to thedonor substrate. In one embodiment, the binder includes 25 to 50%thermoplastic resin by non-volatile weight, and, preferably, 30 to 45%thermoplastic resin by non-volatile weight, although lower amounts ofthermoplastic resin may be used (e.g., 1 to 15 wt. %). The thermoplasticresin is typically chosen to be compatible (i.e., form a one-phasecombination) with the other materials of the binder. A solubilityparameter can be used to indicate compatibility, Polymer Handbook, J.Brandrup, ed., pp. VII 519-557 (1989). In at least some embodiments, athermoplastic resin that has a solubility parameter in the range of 9 to13 (cal/cm³)^(½), preferably, 9.5 to 12 (cal/cm³)^(½), is chosen for thebinder. Examples of suitable thermoplastic resins include polyacrylics,styrene-acrylic polymers and resins, and polyvinyl butyral.

[0058] Conventional coating aids, such as surfactants and dispersingagents, may be added to facilitate the coating process. The LTHC layermay be coated onto the donor substrate using a variety of coatingmethods known in the art. A polymeric or organic LTHC layer is coated,in at least some instances, to a thickness of 0.05 μm to 20 μm,preferably, 0.5 μm to 10 μm, and, more preferably, 1 μm to 7 μm. Aninorganic LTHC layer is coated, in at least some instances, to athickness in the range of 0.001 to 10 μm, and preferably, 0.002 to 1 μm.

[0059] An optional interlayer may be disposed in the donor elementbetween the donor substrate and the transfer layer to minimize damageand contamination of the transferred portion of the transfer layer andto reduce distortion in the transferred portion of the transfer layer.The interlayer may also influence the adhesion of the transfer layer tothe rest of the donor element. Typically, the interlayer has highthermal resistance. The interlayer typically remains in contact with theLTHC layer during the transfer process and is not substantiallytransferred with the transfer layer.

[0060] Suitable interlayers include, for example, polymer films, metallayers (e.g., vapor deposited metal layers), inorganic layers (e.g.,sol-gel deposited layers and vapor deposited layers of inorganic oxides(e.g., silica, titania, and other metal oxides)), and organic/inorganiccomposite layers. Organic materials suitable as interlayer materialsinclude both thermoset and thermoplastic materials, and are preferablycoated on the donor element between the LTHC layer and the transferlayer. Coated interlayers can be formed by conventional coatingprocesses such as solvent coating, extrusion coating, gravure coating,and the like. Suitable thermoset materials include resins that may becrosslinked by heat, radiation, or chemical treatment including, but notlimited to, crosslinked or crosslinkable polyacrylates,polymethacrylates, polyesters, epoxies, polyurethanes, and acrylate andmethacrylate co-polymers. The thermoset materials may be coated onto theLTHC layer as, for example, thermoplastic precursors and subsequentlycrosslinked to form a crosslinked interlayer.

[0061] Suitable thermoplastic materials include, for example,polyacrylates, polymethacrylates, polystyrenes, polyurethanes,polysulfones, polyesters, and polyimides. These thermoplastic organicmaterials may be applied via conventional coating techniques (forexample, solvent coating, spray coating, or extrusion coating).Typically, the glass transition temperature (T_(g)) of thermoplasticmaterials suitable for use in the interlayer is about 25° C. or greater,preferably 50° C. or greater, more preferably 100° C. or greater, andeven more preferably 150° C. or greater. In an exemplary embodiment, theinterlayer has a T_(g) that is greater that a temperature attained inthe transfer layer during imaging. The interlayer may be eithertransmissive, absorbing, reflective, or some combination thereof, at theimaging radiation wavelength.

[0062] Inorganic materials suitable as interlayer materials include, forexample, metals, metal oxides, metal sulfides, and inorganic carboncoatings, including those materials that are highly transmissive orreflective at the imaging light wavelength. These materials may beapplied to the light-to-heat-conversion layer via conventionaltechniques (e.g., vacuum sputtering, vacuum evaporation, or plasma jetdeposition).

[0063] The interlayer may provide a number of benefits. The interlayermay be a barrier against the transfer of material from the LTHC layer.It may also modulate the temperature attained in the transfer layer sothat thermally unstable materials can be transferred. For example, theinterlayer can act as a thermal diffuser to control the temperature atthe interface between the interlayer and the transfer layer relative tothe temperature attained in the LTHC layer. This can improve the quality(i.e., surface roughness, edge roughness, etc.) of the transferredlayer.

[0064] The interlayer may contain additives, including, for example,photoinitiators, surfactants, pigments, plasticizers, and coating aids.The thickness of the interlayer may depend on factors such as, forexample, the material of the interlayer, the material of the LTHC layer,the material of the transfer layer, the wavelength of the imagingradiation, and the duration of exposure of the donor element to imagingradiation. For polymer interlayers, the thickness of the interlayertypically is in the range of 0.05 μm to 10 μm, preferably, from about0.1 μm to 4 μm, more preferably, 0.5 to 3 μm, and, most preferably, 0.8to 2 μm. For inorganic interlayers (e.g., metal or metal compoundinterlayers), the thickness of the interlayer typically is in the rangeof 0.005 μm to 10 μm, preferably, from about 0.01 μm to 3 μm, and, morepreferably, from about 0.02 to 1 μm.

[0065] Table III indicates an exemplary solution for coating aninterlayer. TABLE III Interlayer Formulation Component Parts by WeightButvar ™ B-98 (polyvinylbutyral resin, available from 0.98 Monsanto, St.Louis, MO) Joncryl ™ 67 (acrylic resin, available from 2.95 S. C.Johnson & Son, Racine, WI) Sartomer ™ SR351 ™ (trimethylolpropanetriacrylate, 15.75 available from Sartomer, Exton, PA) Irgacure ™ 369(photocuring agent, available 1.38 from Ciba Specialty Chemicals,Tarrytown, NY) Irgacure ™ 184 (photocuring agent, available 0.2 fromCiba Specialty Chemicals, Tarrytown, NY) 1-methoxy-2-propanol 31.5methyl ethyl ketone 47.24

[0066] The transfer layer, or colorant layer, of the donor element caninclude an organic or inorganic colorant in an organic or inorganiccrosslinkable composition. The colorant generally includes pigments,dyes, or inks, generally disposed in a binder. The transfer layer canoptionally include a surfactant and other additives. Other additives mayalso be included such as an IR absorber, dispersing agents, surfactants,stabilizers, plasticizers, crosslinking agents and coating aids. Thecolorant layer may also contain a variety of additives including but notlimited to dyes, plasticizers, UV stabilizers, film forming additives,and adhesives. Suitable dispersing resins include vinyl chloride/vinylacetate copolymers, poly(vinyl acetate)/crotonic acid copolymers,polyurethanes, styrene maleic anhydride half ester resins,(meth)acrylate polymers and copolymers, poly(vinyl acetals), poly(vinylacetals) modified with anhydrides and amines, hydroxy alkyl celluloseresins and styrene acrylic resins. An exemplary color transfer layercomposition comprises 5-80% by weight colorant, 15-95% by weight resin,and 0-80% by weight crosslinking agent, dispersing agents, andadditives.

[0067] Exemplary pigments and dyes include those listed as having goodcolor permanency and transparency in the NPIRI Raw Materials DataHandbook, Volume 4 (Pigments). Examples of suitable transparentcolorants include Ciba-Geigy Cromophtal Red A2B™, Dainich-SeikaECY-204™, Zeneca Monastral Green 6Y-CL™, and BASF Heliogen Blue L6700F™.Other suitable transparent colorants include Sun RS Magenta 234-007™,Hoechst GS Yellow GG 11-1200™, Sun GS Cyan 249-0592™, Sun RS Cyan248-061, Ciba-Geigy BS Magenta RT-333D™, Ciba-Geigy Microlith Yellow3G-WA™, Ciba-Geigy Microlith Yellow 2R-WA™, Ciba-Geigy Microlith BlueYG-WA™, Ciba-Geigy Microlith Black C-WA™, Ciba-Geigy Microlith VioletRL-WA™, Ciba-Geigy Microlith Red RBS-WA™, any of the Heucotech Aquis II™series, any of the Heucosperse Aquis III™ series, and the like. Anotherclass of pigments than can be used for color filter materials in thepresent invention are various latent pigments such as those availablefrom Ciba-Geigy.

[0068] Either non-aqueous or aqueous dispersions of a colorant in abinder may be used. In the non-aqueous case, solvent-based colorantdispersions may be used along with an appropriate solvent based binder(i.e. Elvacite™ acrylic resins available from ICI Chemicals). It canoften be useful to employ an aqueous dispersion of colorant in binder.In this case, exemplary colorants include pigments in the form ofbinderless aqueous dispersions (i.e. Aquis II™ supplied by Heucotech)and exemplary binders include those specifically designed for pigmentwetting (i.e. Neocryl BT™ acrylic resins from Zeneca Resins). The use ofappropriate binders can promote the formation of sharp, well definededges of the color filters during transfer. When the colorant transferis induced by a high powered light source (i.e., xenon flash lamp), itmay be helpful to include as a binder an energetic or gas producingpolymer such as disclosed in U.S. Pat. Nos. 5,308,737 and 5,278,023.

[0069] The binder system includes organic and/or inorganic polymerizableand/or crosslinkable materials (i.e., monomers, oligomers, prepolymers,and/or polymers), and optionally an initiator system. Using monomers oroligomers can assist in reducing binder cohesive forces in the transferlayer, therefore improving imaging sensitivity and/or transferred imageresolution.

[0070] The colors used to form the color filters are generally primaryadditive colors (i.e. red, green, and blue), and can also be primarysubtractive color (e.g., cyan, magenta, and yellow). Each of theseprimary colors can have high color purity and transmittance, and, whencombined, an appropriate white balance. For use in full color liquidcrystal displays, exemplary color filters can have spectralcharacteristics of red, green, and blue, for example, that showchromaticity close to the National Television Standards Committee (NTSC)standard colors indicated by the Commission International de l'Eclairage(CIE) chromaticity diagram.

[0071] The optional transfer assist layer is typically a layer ofadhesive coated as the outermost layer of the donor sheet. The adhesiveserves to promote complete transfer of the colorant, especially duringthe separation of the donor from the receptor substrate after imaging.Exemplary transfer assist layers include colorless, transparentmaterials with a slight tack or no tack at room temperature, such as thefamily of resins sold by ICI Chemicals under the trade designationElvacite™ (e.g., Elvacite™ 2776). Transfer assist layers can also beoptionally disposed on the receptor.

[0072] Referring again to FIG. 3, after transferring multiple colorfilters to a display substrate and prior to crosslinking the transferredcolor filters, the color filters can be inspected for proper alignment,undesired defects, and the like. Inspection can take place using anysuitable method of inspection. After an optional inspection and prior tocrosslinking, the color filters can be removed for reworking of thedisplay substrate. Removal of the color filters can be performed in anysuitable manner. Typically, the color filters are removed beforecrosslinking by contact with a washing solution in which the colorfilter material is soluble. The color filters can be contacted with anysuitable washing solution using any suitable method including spraying,immersing, and wiping. The particular washing solution used generallydepends on the material of the binder and colorant in the color filterlayer. For example, washing solutions suitable for removing someacrylate-based color filters prior to crosslinking include diluteammonia, sodium hydroxide, various alcohols and ketones, and other suchsuitable solvents.

[0073] Removal of the color filters for reworking of the substrate canbe particularly advantageous when using active display substrates.Active display substrates have a plurality of devices and can be arelatively expensive component in the display. Thus, the ability torework an active substrate after removal of color filters can limitwaste and reduce costs.

[0074] Referring again to FIG. 1, active layer 214 is commonly providedon bottom substrate 212 as shown. In other embodiments, active layer 214can be provided on top substrate 226. For the sake of clarity, theactive layer has been described throughout this document as beingdisposed on the bottom substrate; however, it is understood that theprovision of an active layer on either or both substrates iscontemplated in the present invention.

[0075] In general, active layer 214 includes a plurality of addressabledevices, such as thin film transistors, and a plurality of address lineselectrically connected to the devices. FIGS. 2(a) and (b) show aparticular embodiment where thin film transistors 20 are arranged in arectangular array, are addressed by address lines 32 and 34, and areconnected to sub-pixel electrodes 12. The sub-pixel electrodes aretypically made from a transparent conductive material, and eachelectrode typically covers an area that corresponds to a sub-pixel inthe display. An exemplary transparent conductive material is indium tinoxide (ITO). Each pixel or sub-pixel of the display is preferablyprovided with one or more active devices, and may also be provided withone or more passive devices, such as capacitors, resistors, and thelike. For example, capacitors can be added to sustain an address signalfor a longer period of time so that the pixels can be refreshed lessoften. This can help increase the overall speed and brightness of thedisplay.

[0076] Active layers can optionally include a black matrix to enhancecontrast in the overall display by providing lines of relatively highoptical density (and preferably relatively low surface reflectivity)that separate pixels or sub-pixels and/or that protect the activedevices from undesired light-induced activation. The black matrix can bedisposed between the active layer and the substrate, within the activelayer, or between the active layer and the liquid crystal layer. Oneembodiment is shown in FIG. 2(b) where black matrix 50 is disposed onthe active layer. Whereas the address lines can be used as a blackmatrix, a higher optical density and lower surface reflectivity canoften be obtained by depositing a separate black matrix material.

[0077] Examples of suitable black matrix materials include metals, suchas chromium, provided as a layer or as particles in a binder; metaloxides, nitrides, sulfides, and the like, including chromium oxides,aluminum oxides, and others provided as a layer or as particles in abinder; other opaque inorganic materials provided as a layer or asparticles in a binder; and organic materials such as carbon black, darkpigments or dyes, or other black colorants disposed in a binder. Anexemplary black matrix has on optical density that is greater than 2,and preferably greater than 2.5.

[0078] A black matrix can be provided on an active substrate using anysuitable method. For example, conventional photolithography techniquescan be used to pattern black matrix on the active substrate. Blackmatrix materials can also be directly patterned on the substrate bydeposition through a mask or by various conventional printing methodsincluding thermal print head and ink jet methods. An exemplary method ofpatterning black matrix on active substrates is by selective thermalmass transfer of black matrix material from a donor element to theactive substrate by selectively exposing the donor element to imagingradiation such as from a laser or a flash lamp, as discussed in moredetail below.

[0079] When black matrix material is disposed adjacent to active layerson active substrates, it may be desirable for the material to combinethe advantages of a high optical density (i.e., 2 or greater) using arelatively thin coating while having a high resistivity (i.e., 1×10¹⁰ohm-cm or greater) that substantially prevents crosstalk betweenadjacent devices through the black matrix. Crosstalk is an undesiredleakage of current or inducement of a signal voltage between devices orelectrodes. Various metal oxides, nitrides, sulfides, etc. can be usedthat provide the benefits of high optical density and high resistivity.An exemplary organic black matrix material that has high optical densityand high resistivity and that is suitable for use on active displaysubstrates is disclosed in co-filed and co-pending U.S. patentapplication 09/______ (corresponding to attorney docket no. 54738USA7A,entitled “Thermal Transfer of a Black Matrix Containing Carbon Black”).The black matrix material there disclosed can be especially useful whenconstructing thin displays because a thin black matrix can be formedusing organic material while maintaining a high optical density and highresistivity.

[0080] An exemplary method of patterning a black matrix onto displaysubstrates for active displays includes selective thermal mass transferof a black matrix material from a donor element. The donor element canbe heated by the application of directed heat on selected portions ofthe donor element. Heat can be generated by using a heating element,converting radiation (e.g., light) to heat, and/or applying anelectrical current to a layer of the donor element. In many instances,thermal transfer using light from, for example, a lamp or laser, isadvantageous because of the accuracy and precision that can often beachieved. Imagewise transfer of material from the donor element to areceptor can be achieved by placing a radiation absorber in the donorelement and selectively radiating the donor element, for example, usinga directed laser beam or a flash lamp shined through a mask. To effecttransfer of material from the donor to the receptor, imaging radiationcan generally be directed through the donor element. In some instances,it may be desirable to direct imaging radiation through the receptor.

[0081] Donor elements suitable for transferring black matrix materialare similar to donor elements suitable for selective thermal masstransfer of color filter material as described above. Exemplary donorelement constructions for transferring black matrix materials aredisclosed in co-filed and co-pending U.S. patent application 09/______(corresponding to attorney docket no. 54738USA7A entitled “ThermalTransfer of a Black Matrix Containing Carbon Black”). One suitable donorelement construction includes a polyester film substrate that has anoptional primer layer, an LTHC layer, a temperature modulationinterlayer, and a black matrix transfer layer. Another suitable donorelement has a two-layer construction including a base film and atransferable layer of black matrix material coated on the base film. Thetransferable layer can function as an LTHC layer in this construction.

[0082] Referring again to FIG. 2(b), a planarization layer 16 can alsobe included on the active substrate 10 a. Due to the many componentsdisposed on the active substrate, the surface profile of the activesubstrate tends to be non-uniform. Without a planarization layer, thenon-uniform surface profile can lead to non-uniform spacings between thetop and bottom substrate. Non-uniform spacings can undesirably affectliquid crystal performance and lead to non-uniform displaycharacteristics. The planarization layer can be used to provide asubstantially planar surface so that a uniform gap between substratescan be maintained. Planarization layer materials are preferablynon-birefringent and substantially transparent to light so that thedisplay performance is not adversely affected. Suitable planarizationlayer materials include various planarization layer and overcoatmaterials known in the art such as the overcoat materials for LCD panelconstruction commercially available from JSR Corporation, Yokkaichi,Japan.

[0083] In a particular embodiment, when the sub-pixel electrodes 12 areformed on the planarization layer 16 as in FIG. 2(b), through holes 28can be formed in the intermediate layers, such as the planarizationlayer 16 and color filters R, G, and B, so that electrical contact canbe made between the sub-pixel electrodes 12 and the drain electrodes 26of transistors 20. Through holes can be made using any suitabletechnique such as etching (e.g., using an etch mask to define throughhole positions) or laser ablation. An exemplary method involves coatinga photoresist material over the color filters and planarization layer,photolithographically creating holes in the resist coating in thepositions where the through holes are to be formed, and etching theexposed portions of the planarization layer and color filters to formthrough holes for connecting the sub-pixel electrodes to the drainelectrodes. The photoresist mask can then be removed.

[0084] Another method involves patterning a curable photoresistplanarization layer to also act as a through hole etch mask. In thisway, the portions of the color filters exposed by the patterned holes inthe planarization layer/etch mask can be etched to complete the throughholes. The photoresist coating can remain after etching to form theplanarization layer.

[0085] In one embodiment, color filter formulations suitable for usewith through hole etching processes include those that have a colorantdispersed in a binder that is soluble in solvents compatible with activematrix display substrates. Examples include color filter materials thathave a colorant dispersed in an alkali soluble resin and a water solublethermal crosslinker. The alkali soluble resin can include an acryliccopolymer that contains an acrylic acid unit or a methacrylic acid unit,and the crosslinker can include a water soluble melamine resin.Illustrative formulations of alkali soluble color filter materials forred, green, and blue color filters are given in Table IV. TABLE IVAlkali Soluble Color Filter Formulations Parts by Material SourceFunction weight Red Color Filter Material Cromophtal Red A2B Ciba-Geigyred pigment 80 ECY-204 Dainich-Seika yellow pigment 20 Disperbyk 161BYK-Chemie dispersant 18.4 Elvacite 2669 ICI acrylic resin 134.3 Cymel370 Mitsui Cytec crosslinker 80.6 Green Color Filter Material MonastralGreen 6Y-CL Zeneca green pigment 80 ECY-204 Dainich-Seika yellow pigment20 Disperbyk 161 BYK-Chemie dispersant 17.4 Elvacite 2669 ICI acrylicresin 121.9 Cymel 370 Mitsui Cytec crosslinker 73.2 Blue Color FilterMaterial Heliogen Blue L6700F BASF blue pigment 100 Disperbyk 161BYK-Chemie dispersant 15 Elvacite 2669 ICI acrylic resin 178.1 Cymel 370Mitsui Cytec crosslinker 106.9

[0086] Referring again to FIGS. 2(a) and (b), transparent sub-pixelelectrodes 12 can be any suitable conductive material that issubstantially transparent to light, and can be patterned using anysuitable patterning method such as conventional photolithography and/orsputter deposition through a mask. Various transparent conductive oxidescan be used, typically indium tin oxide (ITO). For application of auniform electric field through the sub-pixel area defined by a sub-pixelelectrode, the electrode is preferably formed on a substantially flatsurface. For example, the sub-pixel electrodes can be formed directly onthe bottom substrate or directly on a planarization layer or otherlayer. FIG. 2(b) shows transparent sub-pixel electrodes 12 formed onplanarization layer 16.

[0087] In a particular embodiment, the liquid crystal material in adisplay of the present invention can be selected to improve displayperformance. Referring again to FIG. 1, liquid crystal layer 220 cancontain any liquid crystal material or mixture of liquid crystalmaterials suitable for electronic display device applications. Commonlyused liquid crystal materials include nematic, chiral nematic, andferroelectric liquid crystals. In general, these materials areclassified according to their mode of operation, which includes theirorientation in a display and behavior upon application of an electricfield.

[0088] Some liquid crystal materials are oriented to wind and unwind inthe plane of the liquid crystal layer. The rotation of the liquidcrystal material causes the polarization of transmitted light to rotateas it passes through the liquid crystal. Application of an electricfield changes the rotation of the liquid crystal material so that lightrays transmitted through activated pixel areas have differently orientedpolarizations than those transmitted through non-activated pixel areas.Some liquid crystal materials are oriented so that their molecules tiltalong the surface of a cone whose symmetry axis lies in the plane of theliquid crystal layer. Upon application of an electric field, the liquidcrystal molecules tilt along the surface of the cone. The orientation ofthe molecules controls the polarization of light transmitted.

[0089] The modes of liquid crystal displays that are most extensivelyemployed at the present time are twisted nematic (TN), supertwistedbirefringence effect (SBE), and dynamic scattering (DS), all employingnematic or chiral nematic (cholesteric) liquid crystals. These devicesare based upon the dielectric alignment effects of the nematic and/orchiral nematic liquid crystal (or mixtures of nematic or chiral nematicliquid crystals) upon application of an electric field. The averagemolecular long axis of the liquid crystal material takes up a preferredorientation in the applied electric field, the orientation of which isdependent on the sign of the dielectric anisotropy of the material ormixture, and this orientation relaxes upon removal of the appliedelectric field. This reorientation and relaxation is slow, on the orderof a few milliseconds.

[0090] Although nematic and chiral nematic liquid crystals are the mostextensively employed, there are liquid crystal devices that employ morehighly ordered smectic liquid crystals. These devices are also based onthe dielectric reorientation of the liquid crystals, and response timesare on the order of milliseconds. A recent advance in the liquid crystalart has been the utilization of tilted chiral smectic liquid crystals,which are also termed ferroelectric liquid crystals (FLC), in deviceswhich give microsecond switching. Accordingly, displays of the presentinvention can employ FLC material to obtain faster switching speeds. FLCmaterials properly aligned in a liquid crystal display can exhibitbistability and symmetric switching characteristics, which can lead toreduced power consumption, as discussed in more detail below with regardto alignment layers.

[0091] In a particular embodiment, displays of the present invention caninclude FLC materials and mixtures thereof. In some embodiments, theliquid crystal material can contain fluorinated chiral FLC compositions.Particularly suited fluorine-containing FLC materials that have recentlybeen developed include those disclosed in U.S. Pat. Nos. 4,886,619;5,082,587; and 5,262,082.

[0092] FLC development has been hindered by problems with defects in theliquid crystal layer structure. These defects arise due to layershrinkage upon cooling (through the temperature ranges associated withthe tilted smectic mesophases) and the resulting formation of a“chevron” layer structure (see, e.g., the discussion by T. P. Rieker etal. in Phys. Rev. Lett. 59, 2658 (1987) and Ferroelectrics 113, 245(1991), as well as the discussion by Y. Ouchi et al. in Jpn. J Appl.Phys. 27, L1993 (1988)). The defects and chevron layer structure oftenresult in a poor contrast ratio and unstable bistability.

[0093] To minimize these defects, admixtures of liquid crystal compoundsthat contract upon cooling through at least one tilted smectic mesophase(positive compounds) and liquid crystal compounds that expand uponcooling through at least one tilted smectic mesophase (negativecompounds) can be prepared to control the liquid crystal layer expansionor contraction behavior, as disclosed in U.S. Pat. No. 5,417,883. Suchmixtures can be used to effect an essentially temperature-independentlayer spacing in the tilted smectic mesophase(s), as well as a reducedtemperature dependence of the layer spacing in the temperature rangeassociated with the transition between the non-tilted and the tiltedmesophases. The process of proper admixing of liquid crystal compoundsto yield a mixture that has complementary expansion and contractionbehavior enables control of layer spacing in the tilted smecticmesophase(s) and thereby control or suppression of chevron layergeometry. Such suppression reduces or eliminates the formation ofzig-zag alignment defects upon cooling through the tilted smecticmesophase(s), resulting in improved electooptical switching performance,increased memory to driven cone-tilt angle ratios, and improved contrastratios. The process also enables the reduction or elimination of thestress-induced defects and non-bookshelf layer geometry which resultfrom heating and cooling cycles.

[0094] Preferred smectic or latent smectic liquid crystal compoundswhich can be utilized in the negative compositions are chiral or achiralliquid crystal compounds which have at least one fluorinated terminalportion and which exhibit negative thermal layer expansion behavior inat least one tilted (or latent tilted) smectic mesophase. A preferredclass of such compounds is that class of chiral or achiral liquidcrystal compounds which have at least one fluoroether terminal portioncontaining at least two catenary, i.e., in-chain, ether oxygen atoms andwhich exhibit negative thermal layer expansion behavior. Such compoundscan comprise, e.g., (a) an aliphatic fluorocarbon terminal portioncontaining at least two catenary ether oxygen atoms; (b) an aliphatichydrocarbon terminal portion; and (c) a central core connecting theterminal portions. The aliphatic hydrocarbon terminal portion of thecompounds can be either chiral or achiral. (Such achiral compounds aredescribed in U.S. Pat. No. 5,262,082 to Janulis et al.)

[0095] Another preferred class of liquid crystal compounds which have atleast one fluorinated terminal portion and which can be utilized in thenegative compositions are those smectic or latent smectic, chiral orachiral liquid crystal compounds which have at least one fluoroaliphaticterminal portion (e.g., the compounds described in U.S. Pat. No.4,886,619 and U.S. Pat. No. 5,082,587 and which exhibit negative thermallayer expansion behavior in at least one tilted (or latent tilted)smectic mesophase. For example,5-hexyl-2-(4′-1,1-dihydroperfluorooctyloxy) phenylpyrimidine has beenfound to exhibit such behavior and to be suitable for such use.

[0096] Other liquid crystal compounds which possess a mechanism whichoffsets the layer shrinkage associated with the tilting of the moleculesin at least one tilted (or latent tilted) smectic mesophase and whichthereby exhibit negative thermal expansion behavior in the mesophase(s)can also be utilized in the negative compositions.

[0097] Liquid crystal compositions suitable for use in admixture withthe above-described negative compositions are those compositions whichhave a net positive thermal layer expansion in at least one tilted (orlatent tilted) smectic mesophase and which comprise at least one smecticor latent smectic liquid crystal compound. Smectic (or latent smectic)liquid crystal compounds suitable for inclusion in the liquid crystalcompositions having net positive thermal expansion behavior (hereinaftertermed “positive compositions”) include chiral and achiral liquidcrystal compounds which have aliphatic terminal portions and whichexhibit positive thermal layer expansion behavior in at least one tilted(or latent tilted) smectic mesophase, e.g., compounds such as the alkyl,alkoxy phenylpyrimidines and the alkoxy, alkoxy phenylbenzoatesdescribed by D. Demus et al., Flussige Kristalle in Tabellen, VEBDeutscher Verlag fur Grundstoffindustrie, pages 65-76 and 260-63,Leipzig (1974) and by S. Kumar, Phys. Rev. A 23, 3207 (1984); chiral andachiral liquid crystal compounds which have at least one fluoroaliphaticterminal portion (such compounds are described, e.g., in U.S. Pat. No.4,886,619 and U.S. Pat. No. 5,082,587) and which exhibit positivethermal layer expansion behavior in at least one tilted (or latenttilted) smectic mesophase; and chiral and achiral liquid crystalcompounds which have at least one fluoroether terminal portioncontaining only one catenary ether oxygen atom and which exhibitpositive thermal layer expansion behavior in at least one tilted (orlatent tilted) smectic mesophase. The latter compounds can be, e.g.,compounds which comprise (a) an aliphatic fluorocarbon terminal portioncontaining one catenary ether oxygen atom; (b) an aliphatic hydrocarbonterminal portion; and (c) a central core connecting the terminalportions.

[0098] When compound(s) that have a fluoroether terminal portioncontaining at least two catenary ether oxygen atoms are used as themajor component(s) of the negative compositions, the compositions canpreferably be combined with positive compositions comprising (as themajor component(s)) at least one chiral or achiral liquid crystalcompound having a fluoroaliphatic terminal portion or, more preferably,a fluoroether terminal portion containing only one catenary ether oxygenatom. Such combinations are preferred from the standpoint ofcompatibility.

[0099] An important characteristics of a liquid crystal display deviceis its response time, i.e., the time required for the device to switchfrom the on (light) state to the off (dark) state. In a ferroelectric oranti-ferroelectric device, response time is proportional to therotational viscosity of the liquid crystal compound(s) contained withinthe device and is inversely proportional to their polarization and tothe applied electric field. Thus, response time can be reduced by usingcompound(s) having high polarizations or low viscosities. Briefly,liquid crystal materials suitable for use in this invention and thatprovide fast response times include fluorine-containing, chiral liquidcrystal compounds having smectic mesophases or latent smecticmesophases. (Compounds having latent smectic mesophases are those whichby themselves do not exhibit a smectic mesophase, but which, when inadmixture with compounds having smectic mesophases or with othercompounds having latent smectic mesophases, develop smectic mesophasesunder appropriate conditions.) Chiral liquid crystal compounds thatprovide fast response times include those materials described in thefollowing U.S. Pat. Nos.: 5,855,812; 5,702,637; 5,658,491; 5,482,650;5,474,705; 5,437,812; 5,417,883; 5,399,291; 5,377,033; 5,262,082;5,254,747; 5,082,587; 5,062,691; and 4,886,619.

[0100] Another advantage that can be obtained from using suitable liquidcrystal mixtures as described above in displays of the present inventionis a low birefringence. The low birefringence of thesefluorine-containing liquid crystal compounds (relative to theirnonfluorine-containing analoques) allows the fabrication of devices withsomewhat larger device spacings without substantial loss of lighttransmission. Light transmission through, e.g., a surface-stabilizedferroelectric device (as described in U.S. Pat. No. 4,367,924) with twopolarizers is represented by the following equation:

I═I_(o) (sin² (4Θ)) (sin² (πΔnd/λ))

[0101] where I_(o) is the transmission through parallel polarizers, Θ isthe liquid crystal material tilt angle, Δn is the liquid crystalbirefringence, d is the device spacing, and λ is the wavelength of lightused. To maximize the transmission, both sin² (4Θ) and sin² (πΔnd/λ)should be at maximized. This occurs when each term equals one. The firstterm is a maximum when the tilt angle equals 22.5°. This is a functionof the liquid crystal and is constant for a given material at a giventemperature. The second term is maximum when Δnd═λ/2. This demonstratesthe criticality of the low birefringence of the materials of thisinvention. Low birefringence allows a larger device thickness, d, for agiven wavelength of light. Thus, a larger device spacing is possiblewhile still maximizing transmission, allowing easier deviceconstruction.

[0102] Referring again to FIG. 1, alignment layers 218 and 222 can beapplied to portions of the interior surfaces of the substrate elementsof the display to cause a desired orientation of the liquid crystalmaterial at the interface of the liquid crystal layer 220. Properalignment of the liquid crystal molecules allows light to be rotatedthrough angles that are complementary to the alignment of the polarizersassociated with the cell. The particular alignment layers used commonlydepends on the type of liquid crystal material used. Examples ofalignment layer materials include certain polyimides, polyamides,polyesters, polysiloxanes, nylon copolymers, organosilsequioxanepolymers, and other suitable polymers capable of being disposed on adisplay substrate and oriented for alignment. Inorganic alignmentlayers, such as silicon oxides, can also be used. A common method oforienting polymeric alignment coatings is by a physical rubbing process.Alignment layers 218 and 222 can be the same or different materials thatprovide symmetric or asymmetric alignment. Particularly usefulasymmetric alignment schemes are described in U.S. Pat. Nos. 5,377,033and 5,831,705.

[0103] When FLC materials are used, alignment layer schemes that providethe FLC layer with good bistability and symmetric switchingcharacteristics can result in displays that have lower powerconsumption. Good bistability and symmetric switching potentials canreduce the need for frequent pixel resetting and/or depolarization dueto charge build up. An alignment layer scheme that may be suitable forthese purposes is disclosed in U.S. Pat. No. 5,831,705, which is whollyincorporated into this document.

[0104] In a particular embodiment, displays of the present inventioninclude spacers selected to improve overall display performance.Referring again to FIG. 1, spacers (not shown) can be provided in theliquid crystal layer 220 to maintain a desired gap between the topsubstrate 226 and bottom substrate 212. For example, FIG. 2(b) showsspacers 42 disposed in liquid crystal layer 40 to maintain a gap betweentop substrate 10 b and bottom substrate 10 a. Control of the spacingsand mechanical forces within the construction of a flat panel displaycan often be critical to the performance of the device, and can dependupon the incorporation of physical spacers into the display. In liquidcrystal displays (LCDs), the polarization of the light exiting thedisplay is controlled in part by the optical path length through theliquid crystal layer. In current display technology, the thickness ofthe liquid crystal layer is determined by spacers, which may be in theform of particles (i.e., spherical beads or fibers), columnar structures(i.e., posts or pillars), microribs, etc.

[0105] A common method for controlling the thickness of the liquidcrystal layer is to deposit a random arrangement of particles having anarrow size distribution over the entire surface of the substrate oralignment layer. This process has an obvious disadvantage in that thereis no control over the placement of the particles resulting in a highpercentage of the particles appearing in the display windows, thusdecreasing the amount of light that may pass through the display. Inmany applications, the particles are not anchored to the substrate andmay shift or migrate causing artifacts to appear in those areas in thedisplay cell. To prevent particle migration, the opposing displaysubstrates are commonly compressed so that the smaller spacer particlesbecome pinned. Generally, thicker glass substrates are preferred towithstand the compression step and to resist local deformation from thelarger spacer particles that are stressed during compression.

[0106] Spacers can also be selectively placed in desired locationsbetween substrate elements in a liquid crystal display, for example byphotolithographic techniques and by thermal mass transfer techniques.These techniques can be used to form highly uniform spacers thattypically include a binder material. The binder material can helpprevent migration of the spacers by adhering to the substrate. Inaddition, the binder material can give more compressibility to thespacers during assembly of the display, thus reducing stresses on theglass substrates during assembly, and can be hardened after assembly tohold the desired spacing. The sizes of the spacers can also be bettercontrolled. These factors can allow thinner glass substrates to be used.

[0107] Selectively placed spacers can also be used as a black matrix.For example, spacers that have a relatively high optical density (e.g.,2 or greater) can be selectively placed in alignment with devices onactive substrates to prevent undesired light induced activation of thedevices. FIG. 2(b) shows a construction where spacers 42 are alignedwith transistors 20.

[0108] An exemplary method for providing uniform spacers in flat paneldisplays is disclosed in U.S. Pat. No. 5,710,097. Spacer elements can beplaced between substrates by selectively irradiating a thermal transferdonor sheet that comprises (a) a support, (b) an optional light-to-heatconversion layer, (c) an optional non-transferable interlayer, (d) atransferable spacer layer and (e) an optional adhesive layer. Theprocess includes the following steps: (i) placing in intimate contact areceptor and the thermal transfer donor sheet described above, (ii)irradiating at least one of the thermal transfer donor sheet or thereceptor (or a portion thereof, i.e., substrate, spacer layer,interlayer, light-to-heat conversion layer, and/or adhesive layer) withimaging radiation to provide sufficient heat in the irradiated areas totransfer the spacer layer to the receptor, and (iii) transferring thetransferable spacer layer in the irradiated areas to the receptor.

[0109] The thermal transfer donor sheet can be prepared by depositinglayers (b), (c), (d) and/or (e) described above onto a support. Thesupport may be constructed of any material known to be useful as asupport for a thermal transfer donor sheet. The support may be either arigid sheet material such as glass or a flexible film. The support maybe smooth or rough, transparent, opaque, translucent, sheet-like ornon-sheet-like. Suitable film supports include polyesters, especiallypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polysulfones, polystyrenes, polycarbonates, polyimides, polyamides,cellulose esters such as, cellulose acetate and cellulose butyrate,polyvinyl chlorides and derivatives thereof, and copolymers comprisingone or more of the above materials. Typical thicknesses of the supportare between about 1 to 200 microns.

[0110] The transferable spacer layer may include organic materials,inorganic materials, or a composite comprising organic materials orinorganic materials that have particles or fibers incorporated therein.Suitable materials include any number of known polymers, copolymers,oligomers and/or monomers. Suitable polymeric binders include materialssuch as thermoset, thermosettable, or thermoplastic polymers, includingphenolic resins (i.e., novolak and resole resins), polyvinylacetates,polyvinylidene chlorides, polyacrylates, cellulose ethers and esters,nitrocelluloses, polycarbonates, polysulfones, polyesters,styrene/acrylonitrile polymers, polystyrenes, cellulose ethers andesters, polyacetals, (meth)acrylate polymers and co-polymers,polyvinylidene chloride, a-chloroacrylonitrile, maleic acid resins andcopolymers, polyimides, poly(amic acids), and poly(amic esters) andmixtures thereof.

[0111] When the transferable spacer layer includes a thermosettablebinder, the thermosettable binder may be crosslinked after transfer tothe receptor. The binder may be crosslinked by any method which isappropriate for that particular thermosettable binder, for example,exposing the thermosettable binder to heat, irradiating with a suitableradiation source, or a chemical curative.

[0112] Particles or fibers may be added to the transferable spacer layerto form a composite. The addition of particles or fibers to thetransferable spacer layer may be accomplished by using any knownparticle or fiber with a spacing dimension less than or equal to thespacing required in the particular display device of interest. Theparticles may have a spacing dimension smaller than the thickness of thetransferable spacer layer or a spacing dimension larger than thethickness of the transferable spacer layer. When the particle size issmaller, the thickness of the transferable spacer layer controls thespacing within the display device. Whereas, when larger particles areused the spacing dimension of the particles used in the compositecontrols the spacing in the display device. Preferably at least 5% ofthe particles have a spacing dimension greater than the thickness of thespacer layer and more preferably at least 10%. Either approach may beused as a means for achieving uniform separation and support of thesubstrates within the display. Suitable particles include organic and/orinorganic materials (solid or hollow) having any suitable shape (i.e.,spheres, rods, posts, triangles, and trapezoids) and size distributionconsistent with maintaining the desired separation. Preferred particlesinclude current LCD spacer spheres, rods, etc. comprised of glass orplastic such as those referenced in Japanese Kokai Patent ApplicationNo. HEI 7[1995]-28068; U.S. Pat. Nos. 4,874,461; 4,983,429; and5,389,288. In LCD displays, it is preferred that the standard deviationfor the size distribution of particles is + or −20% of the mean particlespacing dimension (i.e., mean diameter of a spherical or cylindricalshaped particle, or average height of a cylindrical shaped particle).More preferably, the standard deviation is + or −10% of the mean. Mostpreferably, the standard deviation is + or −5% of the mean. When a fiberis used, the dimensions are typically measured as the denier (orfineness) of the fiber.

[0113] Dispersants, surfactants and other additives (i.e., antioxidants,light stabilizers, and coating aides) may be included to aide in thedispersion of the particles and/or fibers or impart other desirableproperties to the transferable spacer layer as known to those skilled inthe art.

[0114] The compressibility of the element bearing the forces in thedisplay (e.g., the particles in the case where the spacer layercomprises particles with a particle spacing dimension greater than thethickness of the transferable spacer layer and the transferable spacerlayer in cases where the spacer layer does not comprise particles withparticle spacing dimensions greater than the thickness of thetransferable spacer layer) should be sufficient to maintain a uniformspacing gap in the corresponding display.

[0115] The receptor for spacer placement may be any flat panel displayelement benefiting from the application of spacers. The spacers can beprecisely placed in the desired locations to avoid optical interferencein the display windows of the display device. The receptor may beoptionally coated with an adhesive topcoat to facilitate the transfer ofthe transferable spacer layer to the receptor.

[0116] Referring again to FIG. 1, bottom substrate 212 and top substrate226 can be any type of substrate suitable for display applications.Substrates suitable for use in transmissive or transflective liquidcrystal displays of the present invention include rigid or flexiblesubstrates that are substantially transmissive to visible light.Non-birefringent substrates are particularly suited. Examples of rigidsubstrates include glass, low temperature polysilicon (LTPS), and rigidplastic. Suitable flexible substrates include substantially clear andtransmissive polymer films. Suitable polymer substrates includepolyester base (e.g., polyethylene terephthalate, polyethylenenaphthalate), polycarbonate resins, polyolefin resins, polyvinyl resins(e.g., polyvinyl chloride, polyvinylidene chloride, polyvinyl acetals,etc.), cellulose ester bases (e.g., cellulose triacetate, celluloseacetate), and other conventional polymeric films used as supports invarious imaging arts. Transparent polymeric film base of 2 to 100 mils(i.e. 0.05 to 2.54 mm) is preferred.

[0117] For glass substrates, a preferred thickness is 0.2 to 2.0 mm. Itis often desirable to use glass substrates that are 1.0 mm thick orless, or even 0.7 mm thick or less. Thinner substrates result in thinnerand lighter weight displays. Certain processing, handling, andassembling conditions, however, may suggest that thicker substrates beused. For example, some assembly conditions may require compression ofthe display assembly to fix the positions of spacers disposed betweenthe substrates. The spacers are distributed to help maintain a uniformgap between the substrates in which the liquid crystal material can bedisposed. Under such assembly conditions, suitable substrates are thosethat can withstand the stresses required to compress the spacers as wellas resist local deformation caused by larger spacers that undergo highercompressive stressing. The competing concerns of thin substrates forlighter displays and thick substrates for reliable handling andprocessing can be balanced to achieve a preferred construction forparticular display dimensions. As described above, an improved displayof the present invention can include spacers that allow reliableassembly of displays that have thinner substrates.

[0118] If the substrate is a polymeric film, it is preferred that thefilm be non-birefringent to substantially prevent interference with theoperation of the display in which it is to be integrated. Exemplarynon-birefringent substrates are polyesters that are solvent cast.Typical examples of these are those derived from polymers consisting orconsisting essentially of repeating, interpolymerized units derived from9,9-bis-(4-hydroxyphenyl)-fluorene and isophthalic acid, terephthalicacid or mixtures thereof, the polymer being sufficiently low in oligomer(i.e., chemical species having molecular weights of about 8000 or less)content to allow formation of a uniform film. This polymer has beendisclosed as one component in a thermal transfer receiving element inU.S. Pat. No. 5,318,938. Another class of non-birefringent substratesare amorphous polyolefins (e.g., those sold under the trade designationZeonex™ from Nippon Zeon Co., Ltd.).

[0119] In particular embodiment of a display of the present invention, abottom polarizer (or combination of polarizers) can be selected toimprove display performance. Referring again to FIG. 1, polarizer 210can be a reflective polarizer, dichroic polarizer, or other suitablepolarizer or combination of polarizers. For back-lit display modes thatuse a light source such as light source 202 to illuminate the displayfrom below the bottom substrate 212, an exemplary polarizer 210 is acombination of a reflective polarizer and a dichroic polarizer. Forexample, FIG. 2(b) shows a combination bottom polarizer 64 that includesa reflective polarizer 60 and a dichroic polarizer 62. An exemplarycombination polarizer is disclosed in International Publication WO95/17691, the disclosure of which is wholly incorporated by referenceinto this document. The reflective and dichroic polarizers can beprovided in close proximity to one another, and are preferably bondedtogether to eliminate the air gap between the polarizers. A particularlysuited reflective polarizer/dichroic polarizer combination is one inwhich a polyvinyl alcohol (PVA) coating is disposed on a multilayerbirefringent reflective polarizer and is concurrently oriented. The PVAis dyed to form a dichroic polarizer coating on the reflectivepolarizer, as disclosed in International Publication WO 95/17691. Thecombination of a dichroic polarizer coated onto a reflective polarizercan provide a particularly thin and efficient polarizer.

[0120] An advantage of including a reflective polarizer in the bottompolarizer of a display of the present invention is that light having anundesired polarization state can be reflected by the reflectivepolarizer and recycled for transmission through the reflectivepolarizer. Recycling of light in this manner can allow more light topass through the display relative to using absorptive polarizers. Thiscan allow brighter displays that use less power. Particularly suitedreflective polarizers include birefringent multilayer reflectivepolarizers as disclosed in U.S. Pat. No. 5,686,979 and in InternationalPublication WO 95/17691 and birefringent diffusely reflective polarizersas disclosed in International Publication WO 97/32226. When birefringentmultilayer reflective polarizers are combined with a dichroic polarizercoating as discussed above, a particularly thin combinationreflective/dichroic polarizer can be obtained that can also enhance thebrightness of the display.

[0121] All of the patents and patent applications cited are incorporatedinto this document in total as if reproduced in full.

[0122] This invention may be suitably practiced in the absence of anyelement not specifically described in this document.

[0123] Various modifications and alterations of this invention will beapparent to one skilled in the art from the description herein withoutdeparting from the scope and spirit of this invention. Accordingly, theinvention is to be defined by the limitations in the claims and anyequivalents thereto.

What is claimed is:
 1. An electronic display comprising: a light source;a polarizer disposed to transmit and polarize a portion of light fromthe light source; and a liquid crystal display panel disposed to utilizepolarized light transmitted by the polarizer, the display panelincluding a bottom substrate, a top substrate spaced a distance apartfrom the bottom substrate, a bistable liquid crystal layer comprising afluorinated chiral ferroelectric liquid crystal material disposedbetween the top and bottom substrates, and a multi-color active layerdisposed between the bottom substrate and the liquid crystal layer,including a plurality of independently addressable active elementselectrically connected to transparent conductive sub-pixel elements anda plurality of thermally transferred color filters that include acolorant in a crosslinked composition, each color filter aligned withone or more of the transparent conductive sub-pixel elements.
 2. Theelectronic display of claim 1 , further comprising a black matrixdisposed on the active substrate to separate sub-pixels, the blackmatrix having an optical density sufficient to provide optical contrastbetween sub-pixels and a resistivity to substantially prevent crosstalkbetween adjacent independently addressable transistors.
 3. Theelectronic display of claim 1 , further comprising a plurality ofspacers disposed between the first and second substrates to maintain asubstantially uniform gap therebetween.
 4. The electronic display ofclaim 3 , wherein the spacers are selectively thermally masstransferred.
 5. The electronic display of claim 1 , wherein thepolarizer includes a birefringent reflective polarizer and a dichroicpolarizer interposed between the reflective polarizer and the bottomsubstrate.
 6. An electronic display comprising: a first transparentsubstrate having a plurality of independently addressable thin filmtransistors disposed thereon, each transistor electrically connected toan associated transparent conductive sub-pixel element; a secondtransparent substrate spaced a distance apart from the first transparentsubstrate; a liquid crystal layer disposed between the first substrateand the second substrate; and a plurality of thermally transferred colorfilters disposed between the first substrate and the liquid crystallayer, the color filters comprising a colorant in a crosslinkedcomposition, each color filter aligned with one or more of thetransparent conductive elements associated with the transistors.
 7. Theelectronic display of claim 6 , further comprising a black matrixdisposed on the first substrate, the black matrix having an opticaldensity sufficient to provide optical contrast between sub-pixels and aresistivity to substantially prevent crosstalk between adjacentindependently addressable transistors.
 8. The electronic display ofclaim 6 , further comprising a plurality of spacers disposed between thefirst and second substrates to maintain a substantially uniform gaptherebetween.
 9. The electronic display of claim 8 , wherein the spacersare disposed to prevent light induced activation of the transistors. 10.The electronic display of claim 6 , further comprising a light sourceand a reflective polarizer disposed to transmit to the liquid crystallayer light from the light source having a first polarization state andto reflect light having a second, orthogonal polarization state backtoward the light source.
 11. A process for making a color displaysubstrate comprising the steps of: providing a display substrate;thermally mass transferring a plurality of color filters to selectedportions of the substrate, each color filter comprising a colorant in acrosslinkable composition; and crosslinking the color filters after thetransferring step.
 12. The process of claim 11 , further comprising thesteps of: inspecting the color filters after the transferring step andprior to the crosslinking step; and performing a washing step, ifnecessary, to remove the color filters from the substrate for reworkingof the active substrate.
 13. The process of claim 11 , wherein the stepof thermally mass transferring a plurality of color filters comprises:providing a donor sheet comprising a base layer, a light to heatconverter, and a transfer layer comprising a colorant in a crosslinkablecomposition; placing the transfer layer proximate to the displaysubstrate; and selectively irradiating portions of the donor sheet tothermally transfer portions of the transfer layer from the donor sheetto the display substrate.
 14. The process of claim 11 , furthercomprising the step of forming a black matrix on the display substrate.15. The process of claim 11 , wherein the display substrate includes aplurality of independently addressable active devices.
 16. The processof claim 15 , further comprising the step of forming through holes inthe color filters to allow electrical connection of transparentconductive sub-pixel electrodes to the independently addressable activedevices.
 17. The process of claim 16 , wherein the step of formingthrough holes in the color filters comprises patterning a photoresist onthe color filters and etching portions of the color filters left exposedby the patterned photoresist.
 18. The process of claim 16 , wherein thestep of forming through holes in the color filters comprises laserablating portions of the color filters.
 19. A process for making adisplay substrate comprising the steps of: providing an active displaysubstrate having a plurality of independently addressable active devicesthereon; thermally transferring a plurality of color filters to theactive substrate, each color filter comprising a colorant in acrosslinkable composition; inspecting the color filters after thetransferring step; performing a washing step, if necessary, to removethe color filters from the active substrate for reworking of the activesubstrate; and crosslinking the color filters after the inspecting step.20. The process of claim 19 , wherein the step of thermally transferringa plurality of color filters comprises: providing a donor sheetcomprising a base layer, a light to heat converter, and a transfer layercomprising a colorant in a crosslinkable composition; placing thetransfer layer proximate to the active substrate; and selectivelyirradiating portions of the donor sheet to thermally transfer portionsof the transfer layer from the donor sheet to the active substrate.